1. Field of the Invention
This invention relates generally to processes for preparing bis(ether anhydrides). In particular, safer and more efficient imidization, nitration, displacement and exchange reactions leading to the preparation of bis(ether anhydrides) are made possible as a result of conducting the imidization step with a liquid alkylamine wherein the alkyl group of the alkylamine preferably contains at least three carbon atoms.
2. Brief Description of the Background Art
Processes for preparing bis(ether anhydrides) are well known. Bis(ether anhydrides) are intermediates used to prepare polyetherimides which are well known components for plastic automobile parts and the like. Biphenol dianhydride and bisphenol A dianhydride are two frequently used intermediates. Processes for preparing such bis(ether anhydrides) can involve four intermediate steps. Those process steps comprise:
(1) an imidization step in which "make-up" n-alkyl phthalimide is synthesized from an alkylamine and phthalic anhydride,
(2) a nitration step in which the n-alkyl phthalimide is nitrated,
(3) a displacement reaction step in which the nitro substituent on the phthalimide ring is displaced and a bisimide is formed, and
(4) a transformation step ("exchange reaction") in which the bisimide is transformed to dianhydride, preferably by reacting the bisimide with phthalic anhydride in the presence of triethylamine and water. The exchange also can be carried out by a process in which the bisimide is hydrolyzed, acidified and dehydrated to form the desired bis(ether anhydride). The resulting product can then be employed to prepare polyetherimides such as ULTEMO polyetherimides commercially available from General Electric Co. See, for example, U.S. Pat. No. 4,020,089 to Markezich (imidization); U.S. Pat. Nos. 4,902,809 to Groenaweg et al. and 4,599,429 to Odle (nitration); U.S. Pat. No. 4,257,953 to Williams, III et al. (displacement); and U.S. Pat. Nos. 3,957,862 and 3,879,428, both to Heath et al. (exchange reaction).
As with many commercial chemical synthesis processes, there remains a need and desire to improve the process from economic, environmental and overall efficiency aspects. We have specifically identified the use of methylamine in the preparation of the "make-up" phthalimide as an aspect of the existing process that, if avoided, could lead to process improvements.
The disadvantage presented by methylamine is twofold. Methylamine is not only toxic, but also its boiling temperature is relatively low, therefore rendering it gaseous at temperatures under which make-up phthalimide is synthesized. As a result, methylamine requires special toxic gas equipment when preparing the "make-up" phthalimide. In addition, at least two condensers are typically employed to condense methyl phthalimide to its more practical liquid form. The liquid condensate which forms in the condenser, however, solidifies on the walls of the condenser and creates blockages. As a result, the condenser is taken off stream and another condenser is utilized while the blocked condenser is heated to remove the residue. The second condenser is similarly removed once it becomes blocked. A safer and more efficient process thus is desired.
In addition, the resulting methyl-derived phthalimide has a high melting temperature and thus must be stored at temperatures of about 133.degree. C. or higher in order to keep the "make-up" phthalimide in liquid phase for further processing.
The overall efficiency of nitrating methyl phthalimide is relatively low because recycling the methyl phthalimide nitrating agent requires a relatively inefficient nitration concentration system. After nitrating the phthalimide ring to form a methylnitrophthalimide, the nitrophthalimide product and the nitrating agent are recovered. N-methyl nitrophthalimide usually is recovered by removing the nitrating agent and solvent via a falling film evaporator. A falling film evaporator, however, is only capable of concentrating N-methyl nitrophthalimide to about 50% solids. Attempts to remove any more nitric acid results in the precipitation of the methylnitrophthalimide product from the solution. As a result, methylnitrophthalimide reaction product solution is quenched into weak nitric acid. The resulting precipitate is washed in countercurrent fashion on a belt filter. In order to be recycled for later nitration, the remaining dilute nitric acid must be recovered in a nitric acid concentrator and sulphuric acid concentrator (NAC/SAC) system. Both the belt filter and NAC/SAC system are quite inefficient as they employ about two pounds of water for every one pound of methylnitrophthalimide produced.
The N-methyl nitrophthalimide product then is dried by partitioning the product from water into toluene. Handling environmentally hazardous organic solvents thus is required. Organic solvents also are required in the displacement reaction which forms the bisimide.
Further problems and inefficiencies are presented when extracting the by-products created by the displacement reaction of methylnitrophthalimides with the salts of sodium bisphenol. These byproducts typically are extracted at temperatures of about 85.degree. C. using alkali solutions at concentrations of about 1 to about 5% alkali. Extractions with such solutions can cause hydrolysis of the desired bisimide product, and as a result, the time, temperature and alkali concentration must be monitored in order to minimize such hydrolysis.
Displacement reactions with N-methyl nitrophthalimides also can be carried out to only about a 22% solids level in the bisimide reaction mixture. If the reaction is allowed to run to a higher solids level, bisimide product precipitates out of the solvent. In addition, substantial amounts of organic solvents are required for processing mixtures at these levels of solids. Running this reaction to a higher, and thus more efficient, solids level thus is difficult and limited.
Inefficiencies also occur in the exchange reaction with bisimides derived from methyl amines. The reaction influent of the preferred exchange reaction comprises bisimide and excess phthalic anhydride and triethylamine, as well as the accompanying transimidization products of both reactants, e.g., bisimide, imide acid, diacid, phthalic acid, and N-methyl phthalimide. By-products typically are removed by extracting the imide products into toluene, thereby driving the equilibrium and leaving only tetra acid salts in the aqueous phase. These acid salts then are stripped of water and triethylamine to leave the desired dianhydride. Methyl-derived bisimides, however, have a limited solubility in toluene, and, therefore, throughput of reaction product solids in the exchange reaction of these bisimides is low, e.g., about 12%. Greater efficiency is desired.